Multilayer ceramic thick film circuits have been used for many years to increase circuit functionality per unit of area. Heretofore, most of the dielectric materials used in multilayer circuits have been conventional monolithic thick film dielectric compositions. These are comprised of finely divided particles of dielectric solids and inorganic binders dispersed in an organic medium. Such thick film materials are usually applied by screen printing, though they may be applied by other means as well. Thick film materials of this type are very important and will continue to be so.
However, recent advances in circuit technology have placed new demands on dielectric materials for this use in that the drive towards finer resolution and higher reliability has placed a premium on smooth, blister-free dielectric surfaces with good hermeticity. Such surfaces would accept fine-resolution thick-film or thin-film conductor lines with a much lower probability of circuit "opens" or circuit "shorts" to other conductor levels. In turn, thick-film conductor lines should not blister upon the further refiring needed to complete the thick-film multilayer, as this loss of adhesion dramatically reduces circuit reliability.
Dielectric blistering can occur upon initial firing of a hermetic, monolithic dielectric layer, especially over copper conductor. Historically, dielectric blistering has been eliminated by compromising the hermeticity of the dielectric layer. On the other hand, conductor blistering, especially copper blistering, occurs upon either initial firing or subsequent refiring of copper over a hermetic, monolithic dielectric layer. Copper blistering can occur even if the copper is buried under subsequent dielectric layers. Once again, the historical solution to copper blistering has been to compromise the hermeticity of the underlying dielectric layer, raising the possibility of short circuits developing through the dielectric layer with time.
It appears that dielectric blistering is primarily caused by premature sintering of the dielectric layer upon initial firing which traps gas generated at the interface between the dielectric layer and the underlying copper layer. On the other hand, it appears that copper blistering is primarily caused by diffusion of glass from the dielectric up into the copper metallization upon refiring. This glass seals over the copper metallization, which leads to copper blistering.
Notwithstanding the effectiveness of the prior art processes for applying a monolithic layer of ceramic dielectric, in many applications there is a strongly unmet need for blister-free ceramic dielectric layers which can be applied by either conventional methods, such as screen printing, or as a laminated film where more exacting properties are required.